26 Sep
2014
26 Sep
'14
12:03 p.m.
On Thu, 25 Sep 2014, drlegendre . wrote:
Now this varies based upon construction of the
resistors, but it's
almost impossible for any typical 2W resistor to fail dead-short - they
can drift up, they can drift down - but a hard-short is just not what we
see. And wirewound parts tend to act like fuses - they simply go red-hot
and open up if they are forced to carry high fault currents.
If the PSU isn't built for rapid over-current shutdown, and the only
other alternative is the 6800uF cap, then I'd put the money on the cap.
Have you checked for build errors, like missing insulators between
boards & standoffs / mounts? The kind where all is well (often for many
years) until the board coating wears-though and creates a short to the
standoff / mount point?
Those resistors are large ceramic wirewounds mounted vertically. I highly
doubt any of those could fail short.
This type of PSU has some form of crowbar circuit, so my money would be on
either overvoltage causing the crowbar to do its job, or a failure in the
crowbar circuit itself. I highly doubt that 6800uF electrolytic capacitor
would fail short, and although that always possible, I'd think one of the
semiconductors would be more likely to fail short.
From a google search, this very problem is apparently
common to this model
of PSU, but I couldn't find anyone who had troubleshooted
it to the point
of finding the actual cause.